It's interesting that they're saying the following on the "upgrade" page:

"The new Lithium battery is fully compatible with all Elinchrom Quadra packs, both original and the newer RX models.

However, some early model Quadra heads may need a small modification to function correctly with the new lithium battery."

Presumably this is why they're saying it's backward compatible. I don't think everyone is going to interpret what they've said the same way. If someone tells me the battery is backward compatible then I'd expect it to mean the whole kit as it was sold as a single unit not parts of it.